Enables (temporarily) adb trace for sharded perf tests.

third_party/android_testrunner/run_command.py

Index: third_party/android_testrunner/run_command.py
diff --git a/third_party/android_testrunner/run_command.py b/third_party/android_testrunner/run_command.py
index 6b84156fe0e549b50a8a0c996e754b67e41cd799..5c82c61210de2e889e4dabf1868e09a323e6d5eb 100644
--- a/third_party/android_testrunner/run_command.py
+++ b/third_party/android_testrunner/run_command.py
@@ -22,6 +22,7 @@ import subprocess
 import tempfile
 import threading
 import time
+import sys
 
 # local imports
 import errors
@@ -93,12 +94,15 @@ def RunOnce(cmd, timeout_time=None, return_output=True, stdin_input=None):
     stdin_dest = subprocess.PIPE
   else:
     stdin_dest = None
+  stderr_dest = subprocess.STDOUT
+  if os.environ.get('ADB_TRACE'):
+    stderr_dest = sys.stdout
   pipe = subprocess.Popen(
       cmd,
       executable='/bin/bash',
       stdin=stdin_dest,
       stdout=output_dest,
-      stderr=subprocess.STDOUT,
+      stderr=stderr_dest,
       shell=True, close_fds=True,
       preexec_fn=lambda: signal.signal(signal.SIGPIPE, signal.SIG_DFL))